PYTHON-1951 Cap python 2 dnspython version at 1.16.x

Author: behackett

setup.py

@@ -320,14 +320,15 @@ def build_extension(self, ext):
 extras_require = {
     'encryption': ['pymongocrypt'],  # For client side field level encryption.
     'snappy': ['python-snappy'],
-    'srv': ["dnspython>=1.16.0,<2.0.0"],
     'zstd': ['zstandard'],
 }
 vi = sys.version_info
 if vi[0] == 2:
     extras_require.update({'tls': ["ipaddress"]})
+    extras_require.update({'srv': ["dnspython>=1.16.0,<1.17.0"]})
 else:
     extras_require.update({'tls': []})
+    extras_require.update({'srv': ["dnspython>=1.16.0,<2.0.0"]})
 if sys.platform == 'win32':
     extras_require['gssapi'] = ["winkerberos>=0.5.0"]
     if vi < (2, 7, 9):